I've used mu metal in a couple of DIY projects with good results. I also covered the transformer inside my Lamm LL2 preamp and cannot hear a difference. Maybe it is shielded well enough already.
About a year ago I wanted to tackle EMI/RFI inside my digital: Metrum Octave DAC, Audiophilleo and purpose-built PC. At the time I was advised to try 3M EMI Absorber AB5100S instead of ERS cloth. My recollection of the reasons were better performance and that ERS cloth, being cloth, exposes us to the risk of loose conductive fibers inside an electric appliance - must be handled with care.
I bought the 3M "paper". It's not paper, really. It's 1mm thick, doesn't bend, and it's heavy. And worked very well for me. The edges are also conductive, so you want to line them with tape. Never used ERS so can't comment on it.
In my DAC, PC and AP I covered 3 faces of the box: one of each parallel faces inside the box. Also applied small pieces around clocks, integrated circuits, etc. around clocks I applied several layers. I wasn't patient enough to try out various combinations of varying layers in different places. But the impact was positive and noticeable. I crafted a box around my PPA USB card, taped it with vinyl tape, tripple-layered on top of it's clock, etc.
Another nice place to try this was in the circuit breaker box, on the inside of the box door, right in front of the circuit breakers themselves.
Very good product. A lot of user input over at JPlay/Computer Audio forum. Many DIY hard-core explorers there.
